Chronology and social significance of the “princely” barrow cemetery in Łęki Małe and the Central European Early Bronze Age

The “princely” barrows of Łęki Małe, Greater Poland are the oldest such monuments within the distribution area of Únětice societies in Central Europe. While in the Circum-Harz group and in Silesia similar rich furnished graves under mounds have appeared as single monuments as early as 1950 BC, Łęki Małe represents a chain of barrows constructed between 2150 BC and 1800 BC. Of the original 14 mounds, only four were preserved well enough that their complex biographies can now be reconstructed. They included ritual activities (before, during, and after the funeral), and also subsequent incursions, including robberies. The long lasting barrow cemetery at Łęki Małe can be linked to a nearby fortified site, Bruszczewo. Together, Łęki Małe and Bruszczewo represent a stable, socially differentiated society that existed for no less than 350–400 years. Therefore, it can be argued that the Early Bronze Age societies of Greater Poland were extremely sustainable in comparison to those of other Únětice regions.


Introduction
The cemetery at Łęki Małe (now Wilanowo, Kamieniec commune, Grodzisk Wielkopolski county, Greater Poland province) (Fig 1) is one of the most important sites of the Central European Early Bronze Age.It consists of large burial mounds of the U ´ne ˇtice phenomenon.
The barrow cemetery at Łęki Małe is a part of the Kościan group, the most northeasterly U ´ne ˇtice unit [1,2].Together with the U ´ne ˇtice fortified settlement at Bruszczewo [3][4][5][6][7][8][9], it formed a regional centre during U ´ne ˇtice times, and provides essential information on metal production and redistribution patterns concerning the entire Kościan group.
What makes this cemetery exceptional is that it is the only site with several burial mounds, called "princely barrows" or "monumental burial mounds", discovered so far within the U ´ne ˇtice distribution area [10].At other U ´ne ˇtice cemetery sites, only single burial mounds were documented.This is the case of the Circum-Harz group (Bornho ¨ck, Helmsdorf, Leubingen) and in Silesia (Kąty Wrocławskie, Szczepankowice).
The first aim of this article is to present new evidence on the chronology of the cemetery at Łęki Małe.We reconstruct the biography of the mounds by combining newest chronometric evidence (radiocarbon dating and dendrochronology) with stratigraphic information and typo-chronological considerations.
As the biography of the Łęki Małe barrows provides new insights into U ´ne ˇtice social, political, and religious practices and perhaps institutions, the results will be discussed in comparison with the other U ´ne ˇtice regions where "princely"barrows have been identified.This is our second objective: to outline the scope of necessary revisions to current models representing the functioning of the U ´ne ˇtice world, as revealed by multidisciplinary studies of Łęki Małe materials.

Site background
According to archival reports [11], the cemetery at Łęki Małe initially consisted of at least fourteen barrows (Fig 1 and Table 1).By the mid-20th century, nine mounds were still visible (see [10] -there is no Barrow IX showed in the figure mentioned) (Fig 2).Five of these mounds were excavated in the 20th century: Barrow II -partly explored in 1933 [12], Barrow I in 1953 [10], Barrow III in 1955 [13], Barrow IV in 1956-1958 [14] and finally Barrow VI in 1970 [15].Archaeological excavation of Barrow VI in 1970 revealed it was largely destroyed in the late 19th century during construction of a local railway line.
Barrow IV is situated in the southeastern-most part of the cemetery and begins the row of approx.ten barrows that stretch north-west along a minor tributary of the Obra River-Mogilnica River, whose name can be roughly translated as "the river of graves".This burial mound is believed to be the oldest of all archaeologically excavated mounds in Łęki Małe [16].
The site where the Early Bronze cemetery was established was used several times, both before and after the period of its existence (Table 2).The earliest remains on the site are attributed to communities of the 5th millennium BC.Then, human occupations continued in the 4th and 3rd millennium BC [17].Corded Ware communities were among the latter.Their traces are both in the form of settlement and funerary remains [18].A grave of the Corded Ware population marks the first use of Łęki Małe for burying the dead [18,19].Slightly later, already in the Early Bronze Age, Barrows III and IV were raised near this grave.
The latest geoarchaeological research revealed that the selection of burial mound location was not merely inspired by a link to an earlier tradition.The cemetery was established within a specific landscape which is the middle Obra River valley.Its form shows segmentation on at least two levels.The dominant segmentation in this area consists of several tunnel valleys (Fig 2A) that cross-cut the upland perpendicularly, creating a series of upland islands [23][24][25].In Table 1.Łęki Małe.Register of barrows according to historical sources.
VI [10,11,15] Partly destroyed during the construction of the railway line at the end of the 19th century; cut by a railway embankment; the remains of the burial mound were excavated in 1970.VII [10,11] The embankment is not marked on the map from 1954; only the location is indicated; currently invisible.VIII [10,11] Partially destroyed.
IX [10,11] No location on the map of 1953.
https://doi.org/10.1371/journal.pone.0300591.t001some places, there are accumulation structures such as eskers which formed within upland islands on the fringe of tunnel valleys.Small erosion-denudation valleys passing into tunnel valleys separate the ends of eskers from uplands; likewise, gently sloping erosive depressions give the uplands an undulating form.These erosion-denudation valleys have outlets, but in most cases are internally drained.Their northwest-southeast orientation reflects the direction of eskers and tunnel valleys, suggesting the relief may have developed due to the large amount of snowmelt water running through the marginal region of the Leszno Phase of the Vistula glaciation.
Barrows at Łęki Małe occupy the top of an esker and its extension towards the south-east: a patch of outwash plain sediments with a continuation as a moraine upland fragment (Fig 2B).In the Early Bronze Age this area was probably detached from farmland and destined exclusively to bury the deceased.From the north and south-west, some small erosion and

I Traces of Danubian societies
Barrow IV: a pottery fragment in the embankment.
Barrow IV: a fragment of the vessel was found in the embankment (quarter IV, depth 4.15 m).Barrow I: pottery was discovered in quarter I "on a secondary deposit" (probably in an embankment).Barrow III: the pit "Y" was located in the second quarter, under the trench "X".Barrow IV: the pits were discovered under the embankment (in quarter I); the FBC ceramics appeared within the central tomb, and fragments of pottery and flint artefacts were also discovered in the embankment.
[ [10][11][12][13]17] III.Barrow I: a trench in the central part of the embankment, visible on the N-S section of the barrow.
Barrow II: disturbances visible on the cross-section of the barrow, in the central part of the barrow, under the dugout dating back to the early Middle Ages.[10,16] https://doi.org/10.1371/journal.pone.0300591.t002 denudation valleys that reach the Mogilnica valley cut off the area where the barrows stand.Not far away from the esker described there is another one which adjoins the former from the west, both surrounded by an outwash patch.These two esker formations, in turn, together with an outwash rise, are intersected by a shallow, northwest-southeast oriented post-glacial erosion outwash depression which also enters the Mogilnica River in its outlet section.Therefore, natural landscape boundaries separated the two esker structures along with an outwash sediment patch and a fragment of loamy upland.However, these boundaries manifested themselves with a different degree of clearness in the landscape.
Field and geomagnetic surveys have shown that the investigated area -except for Łęki Małe cemetery -was almost free from domestic and ritual activities in the Neolithic and Bronze Age periods.Thus, we interpret the area of approx.14 km 2 as a space devoid of almost all traces of human occupation in the Early Bronze Age (Fig 3).

Methodology applied
In carrying out the tasks undertaken, we used a combination of several methods.First of all, we examined old graphic and written documentation to reconstruct stratigraphy of excavated barrows.A re-analysis of all the original drawings, plans and research diaries kept in the Scientific Archive of the Archaeological Museum in Poznań (AMP), and all printed reports [10,[12][13][14] provided the starting point for chronometric studies based on 14 C dating and dendrochronology.
All organic material (wood, bones, charcoal) from the Łęki Małe burial mounds stored in the AMP collection and not subjected to conservation works was considered suitable for the radiocarbon analyses.Altogether, 70 samples were submitted to the Poznań Radiocarbon Laboratory, Poland [26], of which three bone samples did not retain collagen and could not be utilized for measurement.The radiocarbon chronology was refined through replication of determinations.The improvement was also based on considerations of the age at which the person died and the limited rate of carbon exchange in the bones in the case of human remains and the age of the tree samples taken in the case of charcoal.Finally, a Bayesian approach was applied with explicit assumptions derived from stratigraphic observations.It was also possible to use results of dendrochronological analysis, which was performed on the relics of wood from Barrow I by the Laboratory of Dendrochronology in the Institute for the Study, Conservation and Restoration of Cultural Heritage of the Nicolaus Copernicus University in Toruń, Poland.
Analyses in this paper also incorporate the results of bio-archaeological research, which implicitly address chronological problems as well.The absolute chronology assessments were supported by results of the typo-chronological studies of pottery and metal artefacts.
Finally, we reconstruct the biography of the mounds by combining our new chronometric evidence with stratigraphic information and typo-chronological considerations.

General model of barrow stratigraphy at Łęki Małe
The four best preserved barrows are numbered I to IV and provided the most comprehensive stratigraphic dataset.We have drawn information about the stratigraphy from original drawings and plans kept in the Scientific Archive of the Archaeological Museum in Poznań (hereinafter: AMP), research diaries, and printed reports [10,[12][13][14].
For barrows no.I, III, and IV, profiles of all quarters have survived in the AMP Scientific Archive so that we can recreate two cross-cut sections.At this point, we would like to draw attention to a few stratigraphic observations that are most relevant to the following investigations.
In the case of the Barrow I, on the north-south profile, there are traces of a trench in the top part of the embankment leading to the central tomb (Fig 4), probably related to its robbery and destruction (Table 2) [10].In Barrow III, above the stone pavement sitting in the central tombs' place one can see curved layers of a ditch that resulted in destroying this tomb [13].We encounter a similar case in Barrow IV; there is a distinct ditch in the uppermost central point of the structure [14].This trench proves without a doubt that the central tomb was plundered and destroyed.
For Barrow II, we have a concise section draft (containing mainly the central part) with the stratigraphic position of main features and finds shown schematically.Moreover, several concentric hollow structures were documented [16], probably remains of a later intrusion (a robbery pit?) partially destroying the central tomb.
By compiling the information about the internal structure of these four mounds, we can construct a generalized list of stratigraphic levels that were formed by similar depositional processes and in which the remains of the deceased and artefacts were deposited, and accompanying features (e.g.hearths) were located (Fig 5A ): I.In the virgin soil (central grave) II.On the surface of the virgin soil III.In the primary humus layer IV.On the surface of the primary humus layer V.In the mound (at various stages of mound construction) VI.Destroying the central tomb

VII. Dug into the mound
In the stratigraphic levels, we were able to distinguish stratigraphic contexts, within which distinct units were identified, including: graves, hearths, accumulations of artefacts etc.
The list of stratigraphic levels can easily be transformed into the Harris matrix (Fig 5B).The categories included in the list have been used to describe the stratigraphy of each burial mound (Fig 6) and to define the stratigraphic position of dated materials.

Special case: Clarification of bio-archaeological results, stratigraphic position and absolute chronology of side burials in Barrow IV
A certain issue which requires clarification is the composition and stratigraphic and chronological position of three side single inhumations excavated in the north-western part of the Barrow IV and named skeletons 1, 2 and 3 [14].They were located on the surface of the virgin soil, i.e. in the stratigraphic level II (Fig 6A).According to the first anthropological report, they contained the following human remains: no. 1 -a child aged 14-16, no. 2 -a male aged 30-40, and no. 3 -a child aged 9-10 [27].
A few years before our research (in 2013) skeleton burials no. 1, 2 and 3 were sampled for aDNA and radiocarbon analysis as part of the RISE project at the University of Gothenburg.Until now only results of examination of Burial 2 (sample RISE431) were published [28]: aDNA confirmed that it was an adult male, and 14 C dating indicated an early chronology of the burial (OxA-27967 3762±27 BP; Table 3).
After an anthropological re-analysis carried out in the new project, Burial 1 was found to contain the remains of a child aged 12-14 years, Burial 2 -female, 25-35 years, and Burial 3child, 9-10 years [32].The remains from all three burials were then re-tested for aDNA in the Christian Albrecht University in Kiel.Burials 1 and 3 were confirmed to contain the remains of male children.In the case of Burial 2, the sex of the deceased was determined to be female.Additionally, the aDNA analyses, the results of which will be discussed in a systematic manner further, showed that all individuals deposited in examined burials were related to each other.Moreover, also aDNA of the male made under RISE shows his relationship to the rest of the deceased.
As discrepancies were found in the bio-archaeological assessment of Burial 2, further examination of the bones was undertaken.It confirmed that the vast majority of the remains belonged to a female, but a few may represent a male.After a detailed and multi-stage re- examination of the skeletal material, it can be concluded that the female skeleton was complete, while the male was represented by the only one bone examined in the RISE project.
Extended 14 C AMS analyses were also carried out, resulting in four determinations for each burial (Table 4).Significantly, the four new dates of the Burial 2 are significantly younger than the OXA-27967 designation.
Thus, the inconsistency of the bio-archaeological results of Burial 2 is accompanied by the inconsistency of the chronometric results.They are so significant that no simple explanation can be found for them, e.g. as a consequence of faulty exploration/storage resulting in mixed remains from different burials or mistakes in radiocarbon and archaeogenetic laboratories.The discussed inconsistencies seem to have revealed a fact that was not observed during the exploration and first anthropological studies: the deposition in Burial 2 of the remains of two individuals.
For this reason, it is appropriate to separate two individuals in Burial 2: • Individual 2a -a female whose body was probably intact in the burial; examined in our project and dated with four 14 C determinations Poz-98602 3695±35 BP; Poz-122904 3680±30 BP; Poz-112931 3670±35 BP; Poz-122901 3690±35 BP (Table 4); • Individual 2b -a male whose only selected bones (from which only one has preserved to date) were deposited with the body of a female; one preserved bone was examined in the RISE project (as sample RISE431) and dated with one 14 C determination OxA-27967 3762 ±27 BP (Table 3).
The outlined problem has important implications for the absolute chronology of the Barrow IV, as Burials 1, 2 and 3 belong to the oldest stratigraphic level (II) in the barrow.Thus, establishing their absolute chronology affects all further considerations.In particular, it must be resolved whether the dating of Individual 2b is crucial in determining the time of deposition of Burial 2 as well as nearby Burials 1 and 3. Following the above presented conclusion, we believe that the OxA-27967 determination does not relate to the chronology of either Burial 2 or two other burials located nearby, but probably belongs to human bones from an older male individual, which accompanied as a secondary deposition the female in Burial 2. Consequently the OxA-27967 determination does not date an activity at Barrow IV.
Thus, the age of the three burials will be determined on the basis of 12 convergent datings from the Poznań Radiocarbon Laboratory (Table 4).The OxA determination will be used only in a limited way.

Absolute chronology: Results of radiocarbon and dendrochronological analyses
Before our project started, ten conventional and one AMS 14 C dates were available for the Łęki Małe cemetery (Table 3).Although some of the conventional dates were measured quite some time ago, all radiocarbon determinations correspond with a period in the late 3rd and the early 2nd millennium BC.
In the new approach altogether 70 samples were submitted to the Poznań Radiocarbon Laboratory [26], of which three bone samples did not retain collagen and could not be utilized for measurement.Successful 14 C AMS analyses were therefore carried out on 67 samples: 66 samples derived from Barrows I-IV and one sample was measured from around Barrow IV (Poz-   96953; 2485±35 BP).The last sample is not considered in this paper.In total, 101 14 C determinations were obtained from the 66 samples, with 31 samples yielding two determinations each and two samples producing three dates each (Table 4).Including the date OxA-27967, the following number of 14 C determinations was made for each burial mound: Barrow I-n = 11, Barrow II-n = 32, Barrow III-n = 22 and Barrow IV-n = 37.In total, this article uses 102 radiocarbon determinations.The stable isotopes values ( 13 C and 15 N) of the samples demonstrate the lack of the reservoir effects (Table 5) and we were able to link each sample to one of the seven stratigraphic levels listed above.Exclusively in the case of Barrow I, the preservation of charred wood from the central tomb allowed dendrochronological analysis and wiggle matching of the 14 C determinations.
In the following section, we will present the results for each barrow individually, beginning with Barrow IV at the south-eastern end of the cemetery and preceding in order to the north-west.

Barrow IV
We measured 19 organic samples from Barrow IV, resulting in 37 radiocarbon determinations (Table 6).The samples were taken from human (n = 7) and animal (n = 4) bones, and charcoal (n = 8).Three of the samples have one 14 C determination; 14 samples have two (double), and in the case of two, three determinations were obtained.Based on the known, precise stratigraphic position of each sample, we established a Harris matrix (Fig 6A ).
The stratigraphically earliest samples belong to human individuals 1, 2a, 2b, and 3 (stratigraphic level II, Table 6); radiocarbon determinations on these samples also represent the oldest dates, with the oldest one obtained for Individual 2b (OxA-27967 3762±27 BP), who, however, probably died some time before part of his remains were placed in Burial 2. Nevertheless, dates from two hearths documented directly above burials (in the stratigraphic levels III and IV) confirm the early dating of these graves, as they "seal" the oldest human activities at the barrow: the burial of individuals.Additional dates represent successive phases of human activity taking place in the mound in the Early Bronze Age (stratigraphic levels V and VI).The stratigraphically latest sample, from the intrusion pit (stratigraphic level VII) destroying the central tomb, also returned the youngest date (Poz-101076 265±30 BP; Poz-121183 250±30 BP; Poz-124558 230±30 BP), supporting our stratigraphic model.This action may also mark the time of the destruction and pillage of Barrow IV.

Barrow III
We measured 16 organic samples from Barrow III, producing 22 radiocarbon determinations (Table 7).Six of the samples have two determinations.The samples include charcoal (n = 10) and animal bones (n = 6).
While we recognize the possibility of an old-wood effect (e.g.[33]) for radiocarbon determinations made on charred wood samples, only charcoal has survived from one of the most important contexts of Barrow III.Stone "pavements", located one below the other (each made up of multiple stones) and found inside the central Tomb A, which were documented during excavation in the 1950s, likely mark the destroyed original stone construction of the central tomb.Unfortunately, no preserved human or animal bones were found in this context [34,35].We therefore included seven charcoal samples collected from seven "pavements" in the radiocarbon measurement (Fig 6B), but identified the species of wood and made adjustments based on the lifespan of these species and the principles described below.Four of the samples of charcoal were oak (Quercus L.) and three were pine (Pinus L.).As a rule, the 14 C determinations for these samples follow the stratigraphic order of the "pavements".However, based on the 14 C determinations, the majority of samples are unlikely to have been related to the construction of the central Tomb A. Only the sample taken from "pavement" XVIII may represent material remaining from the central Tomb A. The earliest date ("pavement" XIX; Poz-97195 4110±35) seems to represent re-deposited Neolithic material (perhaps from stage III; see Table 2).The rest of the dates from "pavements" tend to be much younger, falling in the first centuries AD.They thus represent the re-use of this barrow in later times, and the youngest date may indicate the time when the central Tomb A was destroyed.Several other features distinguished during the excavations, namely Hearth no. 1, Hearth no. 2, and Grave B, were also dated using charcoal samples: birch (Betula L.; n = 1) and pine (Pinus L., n = 2).We argue that wood burnt in both hearths was unlikely to have been from trunks of trees that grew for many years, allowing us to exclude the old-wood effect.The two hearths belong to stratigraphic level IV, the surface of the primary humus.Grave B was embedded in the mound (stratigraphic level V), and its bottom touched (but did not cut) the surface of the primary humus.The features revealed in stratigraphic levels IV and V form a microsequence, with the hearths representing the stage of ritual activities that could have been held while the barrow was being constructed.
Six (n = 6) animal bones were found in the mound (stratigraphic level V) (Table 7).One Canis familiaris (dog) bone returned a modern date, while the other five bones date to the Early Bronze Age.

Barrow II
The excavated area covered only the central portion of the barrow, likely including the central tomb, as indicated by partly preserved rounded remnants of a stone structure in the northern part of the unit marking the tomb's extent.The excavation in question reveals, above all, the extent of damage to the central tomb (stratigraphic level VI, Fig 6C).A total of 21 samples were analyzed: 18 of human bone and three of animal bone.11 of the samples produced two 14 C determinations.Thus, there are a total of 32 determinations, of which solely three dates point to a period other than the Early Bronze Age (Table 8).
The human remains derive from the area of the destroyed central tomb.However, they are unlikely to include the remains of the person for whom the tomb was raised, although this individual may be included in the second group discussed below.The human remains can be divided into two categories: individuals preserved as a group represented by dozens of bones (probably in situ) and individuals that have been identified by individual bones (possibly redeposited).Anthropological analysis revealed that the first of these groups includes bones of children and young individuals: two Infans I (3-4 and 6-7 years old), two Infans II (7-8 years old) and a female 20-25 years old.The individuals identified through individual bones are all adults, though their sex and age at death unfortunately could not be determined.We argue that the remains belonging to both of these groups may represent people who were buried immediately following the interment of the central individual, during the initial construction of the barrow.
However, a robbery distorted stratigraphic observations and some bones with Early Bronze Age dates could also document use of Barrow II after its construction by the U ´ne ˇtice community (stratigraphic level V?).This concern mainly the 14 C dates from single bones which were most likely moved as a result of post-depositional processes.

Barrow I
All radiocarbon determinations for Barrow I (Table 9) were measured on wood samples, and were meant to assist the dendrochronological analysis.Unfortunately, the dendrochronology did not provide absolute dating.The inner part of the central tomb of Barrow I was made of oak (Quercus L.) (Fig 6D).Unlike in the case of the hearths from Barrow III, oak used for construction could have been selected from older trees with larger trunks.Dendrological observations did not reveal any bark or sapwood which allows us to suppose that the entire series of dates for Barrow I is loaded with the old wood effect.Likewise, the context of the 14 C determination performed earlier in Michigan and Groningen laboratories (Table 3) cannot be determined.

Integrating stratigraphy, radiocarbon and dendrochronology of barrows
We subjected the radiocarbon determinations and dendrochronological results to a detailed analysis of the mounds' stratigraphy.

Barrow IV
Table 6 presents our proposed improvements to the dating of samples based on considerations of the age at which an individual died and the limited rate of carbon exchange in bones [36] for the case of human bones and the age of the sampled trees in the case of charcoal.We argue that oak (Quercus L.) charcoals from hearths likely represent newly growing trees less than 50 years old.However, oak charcoals not clearly from hearth contexts may represent the remnants of timber constructions; these trees may have been up to 300 years old at the time of felling.We estimate other wood species (willow, pine) to have a short lifespan (up to 30 years).
As the first step in recalculating the data, we took the average of dates received from samples of the same material (Table 6) providing, in turn, the basis for calibration (Fig 7).R_Combine stands for calibration of the mean 14 C ages measured for the same sample (wood or human or animal bones from the same individual).The calibration was computed with the aid of the program OxCal v. 4.2.3 [37,38].
We then made adjustments to the lifespan of organic material in samples, as discussed above (cf.Table 4).For wood samples, we applied an approach analogous to that proposed by Makarowicz et al. [39].  of dates of wood and bone samples, and the dates of tree felling or death of the individuals (as the blue background).Subsequently, we combined the modified dates with our stratigraphic model using Bayesian modeling (Fig 9).The recalculated dates suggest that Barrow IV was formed during the Early Bronze Age, with three phases defined in absolute chronological terms: the instant of barrow's creation -2130-2120 BC, the period of its use in the Early Bronze Age -from the early 21st to the early 19th century, and the moment of the central tomb's destruction -1800/1775 BC.
Phase 1 is based on the dates for the interred individuals.The dating for Individual 2b (OxA-27967) was slightly older than the determinations for Individuals 1, 2a and 3 (Table 6 and Fig 10).According to above considerations (see above: section on the special case of bioarchaeological results from barrow IV) Individual 2b was a male who died earlier and may have been one generation older than the rest of deceased buried in side graves in Barrow IV (Individuals 1, 2a and 3).The date of death of Individual 2b is the earliest in the cemetery, however, it must be considered that his burial could have taken place also at a later date, alongside those of Individuals 1, 2a and 3 (single bones attached to burial 2a?).In this case, it could be assumed, based on stratigraphy, that the graves have been created as a result of a one-time action involving the erection of the barrow dated to ca. 2130-2120 BC.Furthermore, this assumption is the basis of the R_Combine function applied to calculate the dates of level II (Fig 9).If these dates are instead grouped using the Phase function, implying a more or less equal distribution of dates over a finite period, the model becomes no longer internally consistent with an A model compliance index significantly lower than 60 [40].
Hearth no. 1 (stratigraphic level III) and 2 (stratigraphic level IV), are both slightly younger, but still remain within the late 22nd century BC.Including an assumption that the date of the sample from level III is significantly older than this from level IV caused model inconsistencies, a problem solved only by allowing the dates of both levels to be identical.The 14 C determinations, therefore, demonstrate that the first stages of formation of the barrow associated with levels II, and then III and IV developed in a short time, here referred to as Phase 1.
Stratigraphic level V marks the next Phase 2 where the barrow was raised and used during the Early Bronze Age.While five 14 C determinations are available for this level, all produce diverging results.Nevertheless, they fall to the Early Bronze Age, from the early 21st until the beginning of the 19th century BC.Around 1800/1775 BC a deep ditch was cut into the center of the mound, destroying the main tomb and marking the last phase when Early Bronze Age communities used Barrow IV.

Barrow III
The first procedure in recalculating the data was to average the duplicate dates (Table 7).Then, the probability distribution (Fig 11) was generated in two versions: without correction and with correction of the wood lifespan for a given sample (the last one is seen against the from which these samples were derived.The time interval between the formation of the dated wood sample and the felling of the tree was assumed to be exponential and to be in the range of 0-30 years (correction "d1"), 0-50 years ("d4") or 0-300 years ("d5").For a bone sample (ŁękiM 1957: 1068) of an individual who died at the age of 30-40 years, the rejuvenation of the calendar date was applied with a value derived from the normal distribution N (15.5).
The oldest dated stratigraphic features are hearths 1 and 2 (stratigraphic level IV), forming a micro-sequence with Grave B (level V).This group is generally dated to the 19th century BC (Table 7).In turn, the construction time of the central tomb is indirectly indicated by a 14 C determination from "pavement" XVIII, that is around 2000 BC (Table 7).All other determinations from level V probably come from a single horse (Equus caballus) individual and can also be placed in the 19th century BC (Table 7).Radiocarbon determinations for the younger "pavements" (XII, VI, V, IV and II) demonstrate a considerable intermingling and their distribution reflects the in-filling of the intrusion pit that destroyed the center of the barrow.The youngest 14 C determination, from sample Poz-96958 1400±30 BP, provides a terminus ante quem for the digging of this pit ca.600/650 AD.
Thus, we can provide dates for three stages in the use of Barrow III.The construction of this burial mound may have taken place as early as 2000 BC, as indicated by oak wood from the "pavement" XVIII we assume comes from the wood used to build the central tomb.A much firmer basis for dating of this event is provided by 14 C determinations from the two hearths we consider remnants of ritual activity associated with the barrow's creation.They indicate the 19th century BC for these activities.This slightly later chronology is also supported by dates for the stratigraphically younger level V (the mound) which indicate the mound was constructed in the 19th century BC.Finally, the most evident signs of Barrow III's destruction can be attributed to ca. 600-650 AD.

Barrow II
As the stratigraphic data from Barrow II is very scarce, and only its central part was excavated, we assume that the 14 C determinations are dating the building and use of the central tomb (Table 8).Following a procedure identical to the previous cases, Bayesian modeling results were obtained (Fig 13A and 13B).They suggest that the building and use of this tomb fall between the 20th and late 19th/early 18th century BC.

Barrow I
In the case of Barrow I, the preservation of charred wood from the central tomb allowed dendrochronological analysis and wiggle matching of the 14 C determinations (Table 9; Fig 14), significantly narrowing the date range for each sample.In the dendrochronological analysis following ranges were obtained: for ŁM_A 1999-1676 BC, for ŁM_B 2024-1950 BC, for ŁM_C 2167-2145 BC and 2087-2040 BC, and for ŁM_D 2054-2016 BC and 1967-1944 BC (all ranges are of 68.2% probability) (Table 10).
However, none of the samples included a bark or waney edge or sapwood, suggesting the impact of the old wood effect.In all likelihood, the number of missing tree rings was different for each sample.Nevertheless, we argue that the examined wood was used as timber to build the tomb chamber and therefore likely was selected from old trees.It was accepted that these were oaks with age up to 300 years (in Central European conditions, trunks of 300-year-old oak trees typically tend to be about 85 centimeters in diameter) [43,44].Based on this assumption, we calculated the probable dates of felling for all trees (Fig 15).In addition, we assume that all trees were felled within one phase as they were all used in the construction of the same monument ("Phase", Fig 16).The resulting Bayesian modeling suggests that the trees were felled around 2000/1900 BC or immediately after.Łęki Małe, Barrow III.Probability distributions of the dates of the wood samples and (against a blue background) the dates of harvesting the trees from which the samples were derived.The time interval between the formation of the dated wood sample and the felling of the tree was assumed to be exponential and to be in the range of 0-30 years (correction "d1"), 0-50 years ("d4") or 0-300 years ("d5").https://doi.org/10.1371/journal.pone.0300591.g011

Chronological sequence of barrows
In summary, using chronometric dating we reconstructed the date of construction of each tomb from each barrow, despite the fact that the earliest dating of the barrows derives from a different context in each case.For Barrow IV the earliest dates are associated with level II; the un-dated level I might be even older.However, the stratigraphic information suggests there was no large time gap between the dated level II burials and the construction of the central tomb.
In Barrow III the situation is more problematic, as the oldest dates derive from fireplaces in level IV.The other dates belong to level VI, which is associated with the looting of the central tomb.In addition, we were able to date contexts from level VI associated with the looting of the central tomb.From other sites (e.g.Gemeinlebar, Franzhausen, Fidva ´r by Vra ´ble) we know that Early Bronze Age looting took place not much after the construction of the central tomb [45][46][47][48][49][50].It is, therefore, likely that the construction of the tomb took place less than two generations before the looting activities.
In Barrow II, 14 C data on the human and animal bones indicate that it could have been built around 2000/1900 BC.For Barrow I we argue that the dated trees were part of the central wooden chamber and thus could be associated with the construction of the central tomb Table 10.Łęki Małe, Barrow I. Results of calibration of the 14 C age of samples: ŁM_A, ŁM_B, ŁM_C and ŁM_D.Wiggle matching modeling: OxCal v 4.4.2[41], calibration curve IntCal 20 [42].

Sample code
Sequence code (OxCal) Calibration of the

The artefacts typo-chronology of Łęki Małe
The typo-chronological analysis of ceramics and metals from the barrows provide additional information regarding their relative chronology.

Ceramic typo-chronology
Table 11 presents the results of the typo-chronological study of the pottery from the barrows.We found typological correlations with almost all phases of the U ´ne ˇtice periodization, except for the oldest proto-U ´ne ˇtice stage.Most types and examples of pottery correspond to the U ´ne ˇtice classic and post-classic phases, ca.2000-1650 BC.Ceramic type was identified following Zich [51] and Kneisel, Schilz [52].
4. The classic/post-classic U ´ne ˇtice phase, types 1A, 3C2 [51].Analogy: Mikulovice, grave 67 [53] (closest to type L), 20B2 [51], GO 05A [52]  We distinguished six chronological/functional groups: https://doi.org/10.1371/journal.pone.0300591.t011 5. The local forms, types 7K1, 7K2 [51] 6.The functional forms, type 10B2 [51], a miniature vessel and a crucible The results mentioned above were used to develop a presence/absence seriation matrix indicating the date of use of each barrow according to the typo-chronological data (Table 12).The pottery from Barrow IV is the earliest, followed by that from Barrow I.The pottery from Barrows II and III is younger and the barrows are close together in time.

Metal typo-chronology
When considering the chronology of metal finds from the site of Łęki Małe, the most important are those from the destroyed Barrow VI and from the graves in Barrow I (Table 13).The largest collection of metal objects comes from Graves A and D in Barrow I, and the objects are diversified both in terms of form and raw materials.In addition to numerous bronze objects, gold rings made of wire with a return coil (Noppenringe) were found in both graves.
In total, seven gold Noppenringe were discovered at the site: two specimens in Barrow III Grave B, two in Barrow I Tomb A, and three in Barrow I Grave D [10,14].This ornament form is characteristic of the U ´ne ˇtice, but recorded also outside its area of strict occurrence [54] where it often signals the presence of a greater amount of "U ´ne ˇtice influences" in material assemblages [55].These finds, due to their extensive distribution throughout Central Europe, differ in chronological position.From the point of view of the examples examined, however, the most important seems to be their presence in the hordes of U ´ne ˇtice, often accompanied by other metal forms also known from Łęki Małe, e.g.eyelet pins (Ösenkopfnadel).Taking into account the available radiocarbon dating of the Ösenkopfnadel (see below), the gold Noppenringe from Łęki Małe likely belong to the classic U ´ne ˇtice phase, i.e. the BA2a period, 2000-1750 BC [53,54].
In principle, all bronze artefacts discovered on the site have a similar chronological position.Typologically, all of the noted forms are associated with the classic U ´ne ˇtice phase.On the basis of the existing information, it seems possible, however, to separate older and younger finds within this phase, and thus also to indicate a potential chronological difference between Tomb A and Grave D from Barrow I.In the collection of finds from Tomb A, the M1a type halberd and the Cypriot pin constitute potential evidence of the earlier (within the classic U ´ne ˇtice phase) chronological position of the Tomb A. The Cypriot pin is a form with a relatively long tradition of use, noted already in the early U ´ne ˇtice stage and in the earlier Nitra stage [56,57].Together with the aforementioned M1a halberd, this set of objects is characteristic for the period around 2100-1950 BC [58].The remaining bronze objects discovered in Tomb A do not allow further refining of the absolute chronology of the burial.It should be emphasized, however, that the Cypriot pin from Tomb A Barrow I is only partially preserved.Its classification to the Cypriot type was determined by the preserved fragment of the wire coil in the upper part of the pin, but the pin head was not preserved.Therefore, this pin could also potentially have been of a form similar to the pin from the Prague-Mis ˇkovice grave 18 [59], radiocarbon dated to approx.1970-1740 BC (95.4% probability) [60] i.e. in the period of the occurrence of the eyelet pins [59].
The two pins found in Grave D Barrow I are the most chronologically sensitive finds.Although both specimens are currently fragmentary, an analysis of the archive drawing documentation allows for careful identification of the two different types of eyelet pins represented.The first specimen was decorated with three grooves in the upper part.There is a preserved "eye" on the head of this pin fragment.The second specimen does not have a preserved "eye", and its upper part is decorated with a herringbone ornament [61].The results of research on the absolute chronology of Early Bronze Age burials equipped with eyelet pins allow us to determine the age of the specimens from Grave D for the period around 1950-1750 BC [53,60].Furthermore, most of the published dating of Czech finds point to the beginning of this period (1950-1850 BC) as the stage of the most frequent use of the eyelet pins [59].
An additional eyelet pin comes from the heavily destroyed Barrow VI.It is an unornamented specimen, preserved in its entirety, with an "eye" stretched between the edges of the pin head [16].Specimens with a similar form are known from a large collection of finds from the Mikulovice site in Czech (23 such pins discovered in 20 graves [53]).The chronological position of the decorated specimens is specific: among the radiocarbon-dated burials equipped with this form of pins, there are both finds dated around 1900 BC, and much later, around 1750 BC [53].Similar results have been obtained for finds from the Circum-Harz group.In this case, based on the analysis of available radiocarbon dates, the existence of three groups of eyelet pins with different chronological placement was suggested [62].The specimen from Barrow VI can be included in the chronologically youngest Group 3 dated to the period 1900-1750 BC [62].
Taking into account the above information, the hypothesis can be put forward that Tomb A Barrow I is older than Grave D Barrow I and that both are older than Barrow VI.As discussed earlier, in the case of Barrow I this interpretation is further supported by both the stratigraphic data and the analysis of the 14 C dating.

Conclusion: Relative chronology of Łęki Małe barrows
The typo-chronology of the pottery in Łęki Małe barrows supports the results obtained from the absolute chronological data.The sequence of the pre-classic-classic phase is visible in the case of Barrow I, while the pre-classic phase is limited to Barrow IV.The spatial shifts from the larger Barrow IV to Barrow I and then to Barrows II and III, could indicate a decisive change in the biography of the cemetery: the richly furnished Barrow I became the new focal area on which the social development was oriented.
The metal finds generally date to the first three centuries of the 2nd millennium BC.They provide the vital evidence that Barrow VI was built later than Barrow I. mounds used to be there [63][64][65][66].This was also the case of the exceptional mound of the Bornho ¨ck, where only the archival sources helped in the reconstruction of a nowadays flat site [67].
Unlike the other sites mentioned above, each of which contains a single burial mound, the Łęki Małe cemetery is composed of more than ten barrows.The discussed data correspond to use over ca.350 years or 14 human generations.In addition to having a long duration of use, burial activities at Łęki Małe begin in the second half of the 22 nd century, much earlier than at other U ´ne ˇtice barrow sites.The earliest Łęki Małe burial mounds predate other known U ´ne ˇtice barrows (Leubingen, Szczepankowice and Kąty Wrocławskie) by at least one and a half century (Table 13).In Greater Poland, not only Łęki Małe Barrows IV, I and II, but also burial mound no.35 in the Krotoszyn Forest [63] confirm the early use of barrows by U ´ne ˇtice communities (Table 14).All other well-dated U ´ne ˇtice "princely" barrows fit within the time range covered by the Łęki Małe barrows, but no other site displays grave mound building activities for longer than one generation.
With the new chronological results for Łęki Małe, the ritual practice of erecting "princely" barrows can no longer be considered only a late U ´ne ˇtice phenomenon, as earlier proposed [68].The oldest Łęki Małe barrow was already erected around 2130 BC.The next youngest known U ´ne ˇtice barrow is from Leubingen and dendrochronological dates place its construction in the mid-20th century BC (Table 14), around seven generations later.
The proposed duration of ritual U ´ne ˇtice practices at Łęki Małe (ca.2125-1775 BC) is contemporary with the U ´ne ˇtice fortified settlement Bruszczewo (ca.2125-1650 BC) located ca. 15 km to the south.J. Mu ¨ller and J. Kneisel argue that Bruszczewo was a regional center, particularly as regards metallurgy [73].The presence of both a fortified center and an extraordinary barrow cemetery strongly indicate that a new socio-political system was established in the Kościan area during the Early Bronze Age.The regional pollen profile from Lake Wonieść suggests a rapid increase in forest clearance at the same time, beginning at the latest by the 22nd century BC [74].
The existence of stratified U ´ne ˇtice socio-political systems has already been suggested.For the Kościan area, a socio-political system with a stratified society was reconstructed by Czebreszuk [75].For the Circum-Harz group an even more differentiated and complex socio-political system was reconstructed wherein "princely"barrows are seen to be indicators of social hierarchies [68,76,77].
The early dating of the Łęki Małe cemetery demonstrates the existence of comparable, complex social structures since U ´ne ˇtice beginnings and its duration at least 14 generations.In contrast to the Circum-Harz group, where the origins of social differentiation (around the 19th-18th century BC) are found in early U ´ne ˇtice development, in Greater Poland the roots of social differentiation stretch back into the preceding Late Neolithic societies.Furthermore, our new early dates for "princely" barrows, coinciding with the very beginning of U ´ne ˇtice's independent development -close the time gap between this type of funeral rite in U ´ne ˇtice and the funeral rite of burial mounds of the Corded Ware phenomenon [68,78,79].
In the Polish Lowlands, the collected 14 C dates indicate the development of the Corded Ware phenomenon lasted until ca.2200 BC [64,80].Thus, in Greater Poland, U ´ne ˇtice communities may have directly adapted the barrow concept from Corded Ware social practices [79].*Cf.additional information concerning 14 C datings see [72]. https://doi.org/10.1371/journal.pone.0300591.t014 Apart from the chronological proximity, there is a continuity of use of the area in Łęki Małe: a flat grave belonging to the late Corded Ware phase was documented and, at the place of Barrow IV, relatively long-lasting activity of the Corded Ware community has been proven [18].Similarly, in the Circum-Harz group the "princely" barrow of Helmsdorf was placed upon a Corded Ware burial mound [81].It is therefore very likely that in both regions the long lasting tradition of using burial mounds to encourage the ideology of the social hierarchy was transferred from Final Neolithic communities to Early Bronze Age societies.

Conclusions
This article focused on new data on the chronology of the cemetery at Łęki Małe.This data provides a much stronger database and rich body of evidence that calls for close analysis and interpretation in order to offer a new model of the European past during the late 3rd and the early 2nd millennium BC.At this point, we will only highlight some of the research areas we plan to focus on in future publications.Łęki Małe has provided significant new insights into the development of U ´ne ˇtice society in Greater Poland.Around 2150 BC, simultaneously with increased agrarian activities, leading to an increased clearing of the landscape [74], and at the same time as the establishment of the fortified settlement of Bruszczewo [73], which represents the regional distribution center for supra-regional products and metal production, the series of large burial mounds in Łęki Małe was established in the landscape.
The uniqueness of the Kościan region can certainly be traced back to its controlling role in the amber exchange between the Baltic region and Central Europe, as well as probably with the Aegean [82,83], and also to its role in the development of metal casting technology.The latter can be seen particularly in the early dating for overlay casting a pin in Bruszczewo [73] and a casting of a hollow core dagger found in Przysieka Polska [65].
The "princely" burial mounds suggest that permanent or stable structures of power (units of political organization) existed in the U ´ne ˇtice world.Previously, it was argued that this type of political organization lasted for at least 150 years (1950-1800 BC).Thanks to our research, we now know that they developed at least twice as long, i.e. about 330 years (2130-1800 BC).This extends the duration of the politically complex U ´ne ˇtice society from six/seven generations, to possibly fourteen generations.The mechanisms that stabilized the political world of U ´ne ˇtice for so many generations should be the subject of in-depth research in the future.
As it is unlikely that the new social display of powerful people in the form of "princely" barrows would have developed autonomously within the Kościan group, these communities probably followed patterns that also emerged in other U ´ne ˇtice regions: especially on the Saale and in Silesia, but probably also in Bohemia and Moravia.In other words, throughout this long period, the structures in question were not only subjects of internal (U ´ne ˇtice) political life.They also influenced other centers of Europe, shaping the political and cultural face of our continent.

Fig 3 .
Fig 3. Łęki Małe.Results of field surveys around the cemetery.Key: Red dots-finds of pottery dated to the Early Bronze Age, black dots-finds of other artefacts dated to other periods of prehistory, Middle Ages or early modern times.https://doi.org/10.1371/journal.pone.0300591.g003

Fig 7 .
Fig 7. Łęki Małe, Barrow IV.The results of the calibration of radiocarbon dates.R_Combine is the calibration of the mean (calculated by Oxcal) over the 14 C ages obtained for the same wood sample or for bone samples from the same skeleton.Modern dates from stratigraphic level VII are not included.https://doi.org/10.1371/journal.pone.0300591.g007

Fig 8 .
Fig 8. Łęki Małe, Barrow IV.Probability distributions of the dates of the wood samples and (against a blue background) the dates of felling (or death of the individual)from which these samples were derived.The time interval between the formation of the dated wood sample and the felling of the tree was assumed to be exponential and to be in the range of 0-30 years (correction "d1"), 0-50 years ("d4") or 0-300 years ("d5").For a bone sample (ŁękiM 1957: 1068) of an individual who died at the age of 30-40 years, the rejuvenation of the calendar date was applied with a value derived from the normal distribution N (15.5).

Fig 9 .
Fig 9. Łęki Małe, Barrow IV.Bayesian date set modeling results.The dates of samples from stratigraphic levels II, III-IV, V and VI were assumed to form an interrupted time sequence.All samples from stratigraphic level II were assumed to represent the same calendar date.https://doi.org/10.1371/journal.pone.0300591.g009

Fig 11 .
Fig 11.Łęki Małe, Barrow III.Probability distributions of the dates of the wood samples and (against a blue background) the dates of harvesting the trees from which the samples were derived.The time interval between the formation of the dated wood sample and the felling of the tree was assumed to be exponential and to be in the range of 0-30 years (correction "d1"), 0-50 years ("d4") or 0-300 years ("d5").

Fig 12 .
Fig 12. Łęki Małe, Barrow III. A. Bayesian date set modeling results.It was assumed that the sampling dates for stratigraphic level IV are identical, and that the dates for samples from stratigraphic level V that are younger than these fall within one phase.The sampling dates for stratigraphic levels VI and VII were calibrated independently.B. Barrow III, as in Fig 12.A, with a horizontal scale covering only the stratigraphic levels IV and V date range.https://doi.org/10.1371/journal.pone.0300591.g012

Fig 13 .
Fig 13.Łęki Małe, Barrow II. A. Bayesian date set modeling results.VIx stratigraphic level sample dates are assumed to be within one phase.The dates of the samples from stratigraphic level VIy were calibrated as independent.B. Barrow II, as in Fig 13.A, with a horizontal scale covering only the VIx stratigraphic level date range.https://doi.org/10.1371/journal.pone.0300591.g013

Fig 14 .
Fig 14.Łęki Małe, Barrow I. Calibration results of radiocarbon dates.The dated samples were bound in four dendrochronological sequences, so the differences in calendar dates of consecutive samples from a given sequence were known.The probability distribution of the date of the youngest increment associated with that sequence ("Last") is also shown for each sequence.https://doi.org/10.1371/journal.pone.0300591.g014 which may have been around 2000/1900 BC.Thus, except for Barrow III, there is a high probability that we caught the earliest construction period for each barrow with deviations that do not contradict the below proposed model.In conclusion, the analysis of absolute chronology (Fig 17) shows Barrow IV was raised first, around 2130-2120 BC.Some 100 years later Barrow I was erected.Shortly thereafter (20-30 years, one generation), Barrow II was built, and probably later (60-70 years, two generations) Barrow III.Early Bronze activities in the cemetery ceased about 1800/1775 BC, as evidenced by the dating of level VI in Barrows IV, II and III (Fig 18).

Fig 15 .Fig 16 .
Fig 15.Łęki Małe, Barrow I. Probability distributions of the dates of the wood samples and (against a blue background) the dates of harvesting the trees from which the samples were derived.The time interval between the formation of the dated sample wood and the felling of the tree was assumed to be exponential and within the range assuming that the trees had a maximum of 300 annual increments.https://doi.org/10.1371/journal.pone.0300591.g015

Fig 17 .
Fig 17.Łęki Małe.Comparison of the dates of the beginning of the use of barrows I-IV.Top: Probability distributions of the calendar dates of the lower bounds of Bayesian chronological models.The burial mounds were ranked according to the median distribution.Bottom: Diagramshowing the probability that the beginning of the use of the burial x (with the date denoted by "t1") is older than that of the burial y (the date denoted by "t2").

Fig 18 .
Fig 18. Łęki Małe.Comparison of the dates of the ends of the use of barrows I-IV.Top: Probability distributions of the calendar dates of the upper bounds of Bayesian chronological models.The burial mounds were ranked according to the median distribution.Bottom: Diagram showing the probability that the end of the use of the burial x (with the date denoted by "t1") is older than that of the burial y (the date denoted by "t2").https://doi.org/10.1371/journal.pone.0300591.g018

Table 2 .
Łęki Małe.Stages of land use in prehistory and the Middle Ages.

Table 4 .
Łęki Małe.List of radiocarbon dates used in the analyses reported in the text.